The present invention relates to a process for the treatment of combustible, solid radioactive wastes, especially wastes containing radionuclides emitting alpha radiation, in which the wastes are oxidized (that is, combusted in the wet state), at elevated temperatures, by a combination of concentrated (more than 16 moles/liter) sulfuric acid and concentrated nitric acid or NO.sub.x. Valuable radioactive materials may be recovered from the resultant solid residue.
The treatment of combustible, solid radioactive wastes is based on the concept of converting such wastes into a noncombustible condition in a minimally hazardous way. A simultaneous, extensive reduction in volume is desirable to make the final, permanent disposal and/or storage of the thus-concentrated radioactive substances simpler and more economical. A normal combustion of, for example, cellulose-containing material, or rubber or synthetic resins in a furnace entails disadvantages in most cases in that part of the radioactive substances is entrained as suspended particles with the smoke, thus requiring special gas cleaning devices to free the evolving gases from the radioactive particles. Quite generally, a certain risk is incurred by the presence of suspended radioactive particles, inherent in practically any open combustion process. Additionally, with such a combustion method, the plutonium contained in the waste materials is converted to sparsely soluble plutonium oxides and plutonium mixed oxides. Attempts have been made to avoid these disadvantages of open combustion by conducting a chemical destruction of the aforementioned wastes.
Using processes known as wet combustion, combustible wastes have been treated with strongly oxidizing acids, such as, for example, nitric acid, or the oxidative properties of concentrated sulfuric acid at elevated temperatures, for example in the proximity of the H.sub.2 SO.sub.4 boiling point, have been exploited. In this connection, oxidation catalysts have been used, such as selenium, as disclosed in German Pat. No. 1,295,724. The use of selenium is disadvantageous, however, since the element is toxic.
In another prior art process, disclosed in U.S. Pat. No. 3,957,676, the solid wastes are allowed to react with concentrated sulfuric acid at a temperature in the range of from 503.degree. K. to 573.degree. K. (230.degree.-300.degree. C.), and simultaneously and/or thereafter, the waste materials subjected to the reaction are brought into contact with concentrated nitric acid or nitrogen oxides. These reactions cause oxidation of the wastes within the reaction liquid. The solid residue thus produced is separated from the liquid, and valuable radioactive materials are recovered from the residue. Thereafter, the residue, free of the valuable matter, is disposed of, and the H.sub.2 SO.sub.4 l and the HNO.sub.3 are recovered and recycled into the process.
It is stated in U.S. Pat. No. 3,957,676 that in laboratory tests, volume reductions of up to a factor of 160 have been attained using this wet combustion process. A very small amount of acid is said to be consumed in the process, if the acid is processed after use, and reused. The oxidizing step in this process is conducted at or in the close proximity of the reflux temperature of sulfuric acid and should be within the temperature range of 503.degree. to 573.degree. K. For lower temperatures, the reaction rate is said to be lower, and although this offers a means to control the reaction, higher temperatures of about 543.degree. K. (270.degree. C.) are generally preferable for a complete reaction. The process can be conducted at or somewhat above atmospheric pressure, which is said to be an advantage in retaining the radioactive contaminants. Although a relatively high temperature (543.degree. K.) is utilized for the oxidizing step, the throughput of material to be combusted in this process per unit time is relatively small. Thus, approximately 81/2 hours are necessary for 100 g. of mixed waste material from the point of introduction of the waste material into the heated, concentrated sulfuric acid to complete oxidation of the waste material.
It is a further disadvantage of this type of wet combustion process that large amounts of energy are required in the reaction of the waste with the concentrated sulfuric acid and in the oxidation with nitric acid. The conventional processes are further disadvantageous in that the reactions exert a relatively high stress on the materials of the reaction vessels, and there is an additional danger of lump formation and/or a partial conglomeration of waste pieces due to melting, factors which at least impede the normal course of the process.